I understand that it was invented in
the 19th Century by some East Angles living in London and incorporates the flag
of St. George with the arms of the Wuffingas Dynasty on a shield in the centre.
The Wuffingas were the "son of Wuffa" the founder of the ruling family of the
East Angles. Their last king was Saint Edmund King and Martyr who was
killed in a particularly nasty way by the Vikings in the 10th Century and was
canonised for his troubles.James Frankcom, 19 September 2005

Except for the special case of Cornwall, which is more an assimilated Celtic nation rather than an English region, the only English regional flag that has had much popular acceptance is that of East Anglia (Norfolk and Suffolk - sometimes extended to include some or all of Cambridgeshire
and Essex), designed in (I think) 1903 or 1905 for the London Society of East Anglians. It is the Cross of St George of England with over the centre of the cross the shield of the traditional arms of East Anglia, blue with three gold crowns. The arms are effectively identical to the small arms of Sweden, from where the East Anglian royal dynasty, the Wuffingas, were supposed to have originated.
David Prothero, 2 June 1999

There is a medieval map of the English "heptarchy", a period where there were seven Anglo-Saxon kingdoms at war with each other. This map, made I believe in the 12th Century after the heptarchy period is illustrated with banners of the kingdoms. Those shown for Essex, Kent and Sussex appear to be very similar to their "county standards" today, while East Anglia has three crowns on a white background, Mercia appears to have a white dragon of some kind.
James Frankcom, 30 July 2001